A technique, known as hard coating, of coating the surface of machine tools, dies or machine parts with a high surface hardness material excellent in wear, welding and/or corrosion resistance has been widely used to provide enhanced cutting performance of machine tools, extended life of dies and improved wear and corrosion resistances of machine parts.
Metal oxycarbides such as chromium, molybdenum or tungsten oxycarbide have been known as a material for the hard surface coating. In particular, chromium oxycarbide is regarded as one noteworthy hard coating material of machine tools, because it has a micro-Vickers hardness of 2800 kgf/mm2 or more, comparatively superior to a micro-Vickers hardness of 2200 kgf/mm2 in titanium nitride, and excellent corrosion resistance/anti-corrosion performance against hydrochloric acid and vitriolic acid, as well as enhanced cutting performance in high-speed, steel throw-away chips.
Such metal oxycarbide coatings have been typically formed through a plasma CVD method or thermal CVD method using metal hexacarbonyls such as Cr (CO)6, W (CO)6 or MO (CO)6 as a raw material. However, metal hexacarbonyls are expensive, and toxic. In addition, most metal hexacarbonyls are solids under normal temperature and pressure, and are typically used through a process of enclosing solid metal hexacarbonyl in a container, sublimating it in the container, and introducing the sublimed metal hexacarbonyl to a reaction chamber through a duct. Thus, it is difficult to maintain the supply of the raw material gas in a constant amount, because the amount of sublimation is reduced as the remaining amount of the raw material decreases. Further, metal oxycarbides are incomplete decomposition products from metal hexacarbonyls, and thereby a soot-like decomposition product is undesirably deposited on the duct and/or the reaction chamber, which disturbs a smooth reaction.
In order to form a metal oxycarbide coating without using harmful metal hexacarbonyls, the inventor previously proposed a method of subjecting a metal substrate to a reactive plasma process under the presence of at least one metal selected from the group consisting of chromium, molybdenum and tungsten, and at least one reactive gas selected from the group consisting of carbon dioxide and carbon monoxide, while maintaining the metal substrate at a temperature in the range of 600 to 780 K, to form a metal oxycarbide coating on the surface of the substrate (Japanese Patent Laid-Open Publication No. 2001-279426).
However, tungsten oxycarbide is inherently less apt to have a sodium chloride crystal structure as compared to chromium oxycarbide or molybdenum oxycarbide, and it is thereby difficult to form a tungsten oxycarbide coating having a desired micro-Vickers hardness of 1600 kgf/mm2 or more through the above method.